Are you using Developer Mode or made changes in it?
NO
Have you disabled/removed all your mods and addons?
Yes
Brief description of the issue:
Hello, I don’t know how to activate the NAV mode in the autopilot. Is there something I have to know or have to do to make this work?
Build Version # when you first started experiencing this issue:
1.30.12.0
It´s been a while since I flew the Twin Otter, but from what I can remember you have to switch to GPS mode if you want the AP to follow the flight plan. I think that was achieved by the CDI button.
But maybe someone else flying regularly can better explain the function of the AP or clearify.
Thank you for your advise. Please, can you point to me wich one is the CDI button on the picture? Would be awesome! I have been looking for it in the manual, but got no hit.
A BIG thank you - you made my day! 
Now it works fine.
